Discrete surface mounted thin film precision resistors are made in staggering numbers and are soldered by automated equipment known as pick-and-place machines onto circuit boards. Discrete surface mounted resistors include a single resistor and are made in a small number of standard sizes, all of which are tiny. For example, one standard size is known as a Model 1206 which is of rectangular shape being 0.120" on the long side and 0.060" on the short side. The pick-and-place machines are designed to accommodate surface mounted thin film resistors of the different standard sizes with a minimum adjustment of the machine. Surface mounted thin film precision resistors are universally used in electronic circuits as opposed to power circuits.
Surface mounted thin film precision resistors include a rectangular ceramic base, conductive terminals wrapping around the short sides of the base, a single resistive element which connects the conductive terminals together on the top side, and a protective coating over the resistor. The standard technique of making surface mounted thin film precision resistors is to start with a ceramic base of sufficient size to make several hundred or several thousand discrete resistors. Successive conductive and resistive layers are sputtered or vacuum deposited on the base in a repetitive pattern so several hundred or several thousand identical resistors are made on one ceramic wafer. The value of each resistor is measured and trimmed with a laser to bring the resistance to a desired value. The ceramic base is scribed with a laser or is diamond sawed to separate the substrate into the several hundred or several thousand of the individual surface mounted resistors.
The vast majority of surface mounted thin film precision resistors are used in pairs as voltage dividers so an input voltage applied across the series of the two resistors is divided into an output voltage appearing across one of them. The desired ratio of the output voltage to the input voltage depends on the design of the particular circuit being made, and the resistors are made of a value to provide the desired output.
Surface mounted resistors of this type must have resistance values within limits dictated by the particular application of the circuit involved. This is commonly expressed in a resistor specification as some percentage, plus or minus, of a predetermined value. Thus, a common resistor specification says that a resistor must have a value of 1500 ohms.+-.0.5%. A resistor made to close tolerances would be .+-.0.1%.
It is not sufficient that the resistance values of the resistors simply be within a given tolerance at room temperature because the temperature at which the circuit operates may vary thereby changing the resistance values in a manner that inherently provides a larger spread of resistance values. Thus, one resistor might have a temperature coefficient of 0.005% ohms per 0.degree. C. rise. Thus, it is necessary that resistors be made to have values which change in response to temperature in a predictable or determinable manner.
It is not sufficient that resistance values of the discrete resistors have temperature coefficients of resistance which can be predicted or determined. Resistance values also change in response to time because the resistor material, the conductors, or both, age in response to factors that are not well understood.
There are a variety of other factors, shown in Table I, which affect resistance values and which, in most applications, must be taken into account:
TABLE I ______________________________________ Factor Affecting Resistance Test Method .DELTA. R/R .DELTA. ratio ______________________________________ thermal shock MIL-STD 202, .+-..02% .+-..005% Method 107 short time overload MIL-R-55342, .+-..02% .+-..005% Par 4.7.5 high temp exposure MIL-R-55342, .+-..03% .+-..01% Par 4.7.6 resistance to bond MIL-R-55342, .+-..01% .+-..01% exposure Par 4.7.7 moisture resistance MIL-STD-202, .+-..03% .+-..02% Method 106 load life MIL-STD-202, .+-..03% .+-..01% Method 108 low temp operation MIL-R-55342, .+-..01% +.005% Par 4.7.4 ______________________________________
These complications in the production of surface mounted thin film precision resistors are sometimes aggravated and sometimes alleviated by the use of resistors in pairs to provide voltage dividers. For example, in paired resistors the absolute resistance values of each resistor might vary substantially so long as their ratio remains close to the desired value. Thus, in a voltage divider, the specification might be 10:1.+-.0.05% indicating the ratio of the input voltage to the output voltage. From elementary circuit theory, the ratio of the two resistors making up the divider network in the above example can be shown to be 9:1+/-0.0555%. In general, the percent error in the output voltage of the divider network is given by: ##EQU1## for the circuit ##STR1## where: R1.degree.=nominal value of resistor 1
R2.degree.=nominal value of resistor 2 PA1 R1=actual value of resistor 1 PA1 R2=actual value of resistor 2 PA1 .DELTA..sub.1 =decimal deviation of resistor 1 from nominal PA1 .DELTA..sub.2 =decimal deviation of resistor 2 from nominal PA1 .DELTA.=.DELTA..sub.2 -.DELTA..sub.1 ##EQU2##
On the other hand, changes in resistance caused by temperature change, aging, thermal shock, short time overload, high temperature exposure, resistance to bond exposure, moisture resistance, load life and low temperature operation makes problems worse. This will be apparent because one of the resistor pair may vary in one direction and the other resistor may vary in the opposite direction. Even where the resistors vary in the same direction, they will normally vary at different rates. Thus, the percentage variation of a pair of resistors can be twice the percentage variation of a single resistor. For example, if a particular resistor has a temperature coefficient of 20 ppm per 0.degree. C. rise, and the second resistor of the divider pair has a temperature coefficient of -20 ppm per .degree.C. rise, then the divider will have a temperature coefficient of 40 ppm per .degree.C. rise. Thus, over a temperature range of 50.degree. C., the voltage will change by approximately 0.2%.
Similarly, if one resistor drifts -0.05% over the life of the resistor and the second by +0.15%, the voltage ratio drift over time will be approximately 0.2%. It will be apparent to those skilled in the art that the divergence in voltage ratio specified in the above example are approximately true when the voltage ratio of the divider is on the order of 10 or more. The exact drift in voltage ratio for a given drift in R1 and R2 is given by equation (1).
To accommodate these factors, resistors have to be made to much more stringent specifications than one might expect. The effort spent in meeting more stringent specifications is reflected in the cost and thus the price of discrete surface mounted thin film precision resistors.
It will be apparent to those skilled in the art that thin film resistors are different from thick film resistors. The resistive and conductive layers on thin film resistors are normally deposited by sputtering or vacuum deposition and typically are on the order of 500-1000 Angstroms or several hundred atomic layers thick. The resistive and conductive layers on thick film resistors are normally applied by screen printing and typically are on the order of 15 micrometers or about 1/2 mil thick. Thick film resistors are typically much larger and carry much greater amperages than thin film resistors. Thick film resistors have much greater variations in resistance, response to temperature changes, aging, thermal shock, short time overload, high temperature exposure, resistance to bond exposure, moisture resistance, load life and low temperature operation. The reasons for these differences become apparent when reflecting on the different resistive materials and the different techniques of applying the conductive and resistive layers to the substrate. Printing a paste onto a substrate is inherently a much cheaper, less complicated, more rugged and less precise exercise than vacuum deposition or sputtering and this results in greater variation in the resistor element. Those skilled in the art know that thick film resistors have their application, thin film resistors have their application and rarely do they overlap.
It is known in the prior art to place multiple resistors on a single device. These devices are known as network or array resistors. Typically, there are one or more input connectors leading to a fair number, usually six or eight, of output connectors. Some network resistors are thick film devices such as shown in U.S. Pat. No. 5,285,184 and some are thin film resistors. For reasons which are largely historical, network resistors are not made in the same manner as thin film precision resistors, their applications rarely overlap and the technologies have evolved independently. For example, thin film precision resistors are used exclusively in electronic circuits, on circuit boards, as voltage dividers. Network resistors, on the other hand, are typically thick film devices used in loose tolerance digital applications.
Of some interest regarding this invention are the disclosures in U.S. Pat. Nos. 4,475,099; 4,505,032; 4,584,553; 5,170,146 and 5,257,005.